Cause then you have to buff them to be equal to their counterpart. This means giving them turbo mortar and pak40 level of AT gun. Also if you make the maxim a 4 model weapon, then you have to spread out the formation so a rifle grenade or any other nade kills at most 2 models AND you have to give it german teleportation capabilities which are not currently implemented in the game (animation).
Finally, the 120mm on the current live version is bad. Specially when mortars get more range at vet3 (which a 120mm will barely hit vet1 and a half due to xp requirements).
Nerfs: increase reinforce cost. The problem is the concept (offensive MG, short arc, fast setup, etc) but it's too late to change it.
I can see your arguemnt for the pak gun/zis gun. As for the "turbo mortar" I'd say making the HM-38 mortar equal to ostheer wouldn't be bad at all, including veterancy. Yes asymmetric design but more or less it would be more balanced. If by turbo mortar you mean USF mortar then we have a problem.
I don't see how the HM-120 is "bad". It comes on very powerful commanders, has a larger AoE than other stock mortars and has a longer range. Vet 3 additional range are those mortars issues, not a soviet mortar issue.
But yes other acceptable nerfs could be reinforce cost and setup/packup time.